zlacker

I've started using Cloudflare's domain register services, and have no complaints so far. I've also used Gandi in the past and they've been good (although a bit more expensive).

>>monkey+(OP)
Seconded cloudflare - basically at-cost registrations, and I already use them for CDN on most domains, so it was a no brainer.

Only downside is their new registrar doesn't support many new-TLDs, so it's a bit of a hybrid setup for some domains.